Add a "Usage with Rollup" section to the readme

[ansballard]

Just wanted to see if here was interest in having a rollup config section along with browserify, webpack, and babel. It's pretty straightforward, just throw in rollup-plugin-alias and alias react and react-dom to dist/preact-compat.es.js.

Submitting from work (PDS Energy).

[developit]

Sure! That definitely seems like a helpful thing to have around. If you're able to submit a PR for it I'd be happy to review :)

[oren]

Maybe we should create a doc folder and inside have 3 folders: tutorials, how-tos, and references. Rollup related documentation can potentially be in any or all of those folders.